<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <link rel="stylesheet" href="css/bootstrap.min.css">
</head>

<style>
  .a{
    height: 50px;
    border: 1px solid black; 
    background-color: #eeeeee; 
  }
</style>

<body>
    <div class="container">
        <div class="row">

            <!-- col-lg-3 当大屏幕时，一个div占3份，一行显示4个div --> 
            <!-- col-md-4 当中屏幕时，一个div占4份，一行显示3个div --> 
            <!-- col-sm-6 当小屏幕时，一个div占6份，一行显示2个div --> 
            <!-- 超小屏幕时，一个div占12份，一行显示1个div，默认 -->

             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>

             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>

             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
             <div class="col-lg-3 col-md-4 col-sm-6 a">x</div>
        </div>
    </div>
</body>
</html>